Negative particle sources consist of means for generating and containing a plasma to provide the charged particles, one or more extraction and accelerating electrodes and a magnetic selector for the particular type of charged particle which it is desired that the source should produce.
Hitherto, the magnetic selectors have taken the form of arrangements of permanent magnets. These have disadvantages in that not only are the field configurations produced by these magnets not ideal for the purpose of suppressing the emission of one type of charged particle in favour of another, but the value of the magnetic field cannot be changed readily, thus restricting any given source to the production of charged particles with a particular charge to mass ratio.